1. Field of the Invention
The present invention relates to a stripper device for a punching tool and a punch assembly using the same stripper device.
2. Description of the Prior Art
FIG. 1 shows a conventional punch assembly used for a punch press. This punch assembly 101 is mainly composed of three parts of a punch body 103, a punch guide 105, and a compression spring 107. The punch body 103 is formed with a punch trunk portion 109, a small-diameter punch head mounting portion 111 formed on an upper side of the punch trunk portion 109, and a cutting edge portion 113 formed on a lower side of the punch trunk portion 109. The punch guide 105 is formed into a hollow cylindrical shape, and further formed with a flange portion 115 and a punch guide hole 117 for guiding the punch trunk portion 109 of the punch body 103 in the axial direction thereof. Further, the punch guide 105 is formed with a key groove 119 on an inner side wall of the punch guide hole 117 so as to extend from the flange portion 115 to near the lower end of the punch guide 105.
A key 121 is attached to the punch body 103 so as to be engaged with the key groove 119 formed in the punch guide 105. Therefore, the punch trunk portion 109 fitted to the punch guide hole 117 of the punch guide 105 is slidably movable only in the axial direction thereof, without rotation under the restriction of the engagement of the key 121 attached to the punch body 103 with the key groove 119 formed in the punch guide 105.
The punch head mounting portion 111 of the punch body 103 projects from the upper end of the punch guide 105. The punch mounting portion 111 of the punch body 103 is formed with a male thread portion 123 at the upper end thereof. A punch head 125 is screwed with this male thread portion 123 of the punch body 103 so that the height of the punch head 125 can be adjusted. Further, the punch head 125 is formed with a flange portion 131 and a lower and outer conical portion 127. A plurality (two or three) of screw holes 133 are formed in the flange portion 131 of the punch head 125 at regular intervals of angle. Further, a flat annular fastening member 129 is fitted to a lower conical portion 127 formed in the punch head 125.
Further, a retainer collar 137 is fitted to a tapered portion 135 formed between the large-diameter punch trunk portion 109 and the small-diameter punch head mounting portion 111 of the punch body 103. The retainer collar 137 is formed with a flange portion 139 at the middle thereof, whose outer diameter is roughly equal to that of the flat annular fastening member 129. The flange portion 139 is in contact with the flange portion 115 of the punch guide 105. An O-ring (sealing member) 141 is interposed between the outer circumference of the cylindrical portion projecting downward from the flange portion 139 of the retainer collar 137 and the inner circumference of the punch guide hole 117 of the punch guide 105 so that the retainer collar 137 can be removably inserted into the punch guide hole 117 of the punch guide 105.
The compression spring 107 is interposed between the flat annular fastening member 129 fitted to the punch head 125 and the flange portion 139 of the retainer collar 137 under a predetermined spring force so as to function as a stripper of the punch body 103. Further, this stripper compression spring 107 can be replaced with another spring such as urethane spring or dish spring, etc.
Further, the punch guide 105 is formed with an outer key groove 143 engaged with a key (not shown) formed in a die holder of a punch press, to decide the angular position of the punch press relative to the die.
The above-mentioned punch assembly (male type) 101 is used together with a mated die (female type) as a pair of punching tool. Therefore, when the punch head 125 is struck by a striker (not shown) of the punch press, plate material can be punched off by the punching tool.
In the above-mentioned conventional punch assembly 101 constructed as described above, when a large stripping Force (or load) is required for the punching, a compression spring 107 with a large spring constant must be used. For instance, when a punching force of 30 ton is required for a plate material with a thickness of 6 mm, the required stripping force for stripping the punch from the punched plate material is as large as 3, 000 kg. In this case, since the required total deformation (compression stroke) of the compression spring 107 is about 10 mm, the spring constant of the compression spring 107 becomes as large as 300 kg/mm.
Accordingly, in order to mount the compression spring 107 as strong as above between the punch head 125 and the retainer collar 137 so as to have a predetermined mounting load (an initial tripping force), a large force is required to deform the compression spring 107 of a large spring constant.
In addition, whenever the cutting edge portion 113 of the punch body 103 is polished, the compression spring 107 must be removed from between the punch head 125 and the retainer collar 137. In this case, however, since the female thread portion of the punch head 125 is strongly engaged with the male thread portion 123 of the punch head mounting portion 111 of the punch body 103 on the basis of a wedge effect of the annular fastening member 129 against the conical portion 127 of the punch head 125, the removal step of the compression spring 107 from the punch assembly 101 is such that: first, two screws are turned into the screw holes 133 formed in the punch head 125 and then the two screws are turned to loosen the annular fastening member 129 away from the compression spring 107, thus causing a problem in that a troublesome work and a strong work force are required for the removal of the punch head 125 from the punch body 103.
Further, after the cutting edge portion 113 has been polished, since the length of the punch body 103 is reduced due to the polishing, although the total length of the punch assembly 101 must be adjusted again, the similar troublesome work and strong removal force are required.
Further, the compression spring 107 is usually designed on the basis of the maximum thickness of the plate material to be punched. Therefore, when a plate material with a relatively small thickness is punched, since a stripping force generated by the compression spring 107 is fairly reduced (because the deformation of the compression spring is small), there arises another problem in that stripping miss occurs frequently. Once the stripping miss occurs, the punch body 103 cannot be removed from the punched plate material.
Furthermore, recently, in order to reduce noise generated during punching processing, there has been developed such a punch press that a hydraulic source is used to drive the punch assembly of the punch press. In this case, since the punching speed is reduced, the area of the secondary shearing surface (described later) inevitably increases, so that the material adheres to the cutting edge portion of the punch due to fusion, thus causing another problem in that the stripping miss easily occurs.
In order to reduce the stripping miss generated when thin material is punched, it may be possible to increase the initial compression force of the compression spring 107 used as a stripper device for a punching tool. In this case, however, since the mounting load of the compression spring 107 increases excessively when thick material is to be punched, the punched products must be pushed by use of additional plate material pushing members, thus causing another problem in that punched products are damaged (nicks or gouges) by the plate material push members. At the same time, since a large internal stress is inevitably generated and concentrated in the compression spring 107, the lifetime of the compression spring is relatively short, and thereby the assembly and disassembly work of the punch assembly are more troublesome and difficult.
Further, as another method of overcoming the above-mentioned problems, it may be possible to use a plurality of small compression springs. In this case, however, since a large compression spring mounting space is required, this method cannot solve the afore-mentioned various problems fundamentally.